1. Field of the Invention
The present invention relates to a solid-state image pickup device of a charge coupled device (CCD) type and a method for driving the solid-state image pickup device, and an electronic apparatus, such as a camera, including the solid-state image pickup device.
2. Description of the Related Art
As solid-state image pickup devices, solid-state image pickup devices using charge coupled elements (CCDs) are in common use. In such a solid-state image pickup device of a CCD type (hereinafter, referred to as a “CCD solid-state image pickup device”), a plurality of light sensing units are arranged in a two-dimensional matrix form. Each of the light sensing units is configured using a photoelectric conversion element that generates signal charge in accordance with the amount of received light and that stores the signal charge, i.e., a photodiode. Light signals from an object enter the photodiodes of the light sensing units. Signal charge is generated in accordance with the light signals, and stored. Each of vertical transfer registers having a CCD structure is provided for a corresponding one of columns of the light sensing units. The signal charge is transferred by the vertical transfer registers in the vertical direction, and transferred by a horizontal transfer register having a CCD structure in the horizontal direction. Then, the signal charge that has been transferred in the horizontal direction is output as image information concerning the object from an output section having a charge-to-voltage conversion unit. Typically, the output section is configured using a so-called floating diffusion amplifier (an FD amplifier) including a floating diffusion (FD) unit, which is connected to a final stage of the horizontal transfer resister, and a source follower amplifier. Further, a so-called floating gate amplifier (an FG amplifier) is in common use as the output section.
The CCD solid-state image pickup devices are broadly used, for example, in digital still cameras or digital video cameras. Furthermore, the CCD solid-state image pickup devices are used as solid-state image pickup devices that are mounted in mobile apparatuses, such as mobile phones with cameras or personal digital assistants (PADs).
In recent years, in the CCD solid-state image pickup devices, as the number of pixels has been increased and the resolution has been increased, the conversion gain of FD amplifiers has been increased. In other words, since the size of pixels has been reduced and the amount of charge that can be handled has been reduced, FD amplifiers having high conversion gains have been used.
On the other hand, a CCD solid-state image pickup device in which the size of pixels is large, the amount of charge that light sensing units, vertical transfer registers, or a horizontal transfer register can handle can be easily set to be large. However, because of constraints to the allowable voltage amplitude of FD amplifiers, an FD amplifier having a reduced conversion gain is used. Typically, FD amplifiers have high conversion gains. However, in the CCD solid-state image pickup device in which the size of pixels is large, because of the above-mentioned constraints, an FD amplifier is mounted, which has a conversion gain that is increased within a range of the allowable voltage of the amplifier.
In order to improve the S/N ratio of a source follower amplifier that is included in the FD amplifier, a configuration is used, in which the mutual conductance (gm) of a drive MOS transistor that is placed at a first stage is increased, and in which the capacitance of a gate unit is reduced. However, in the CCD solid-state image pickup device in which the size of pixels is large and in which the amount of charge that can be handled is large, the capacitance of a floating diffusion unit or the gate unit is set to be larger, so that the conversion gain of the FD amplifier is reduced. As a result, the S/N ratio is sacrificed in a case of low illuminance.
Hitherto, as CCD solid-state image pickup devices that can support both a small signal and a large signal, some CCD solid-state image pickup devices having a configuration in which a capacitor is added to a floating diffusion unit have been proposed. Regarding the operation principal, either a small capacitor or a large capacitor can be set in the floating diffusion unit. The floating diffusion unit has a high conversion gain when the small capacitor is set, and has a low conversion gain when the large capacitor is set. With the high conversion gain and the low conversion gain, a case in which a small signal is input and a case in which a large signal is input are supported, respectively. In this case, the conversion gain is switched in accordance with determination using a signal of the previous frame or the like. Alternatively, the conversion gain is manually switched.
In Japanese Unexamined Patent Application Publication No. 7-79385, a CCD solid-state image pickup device is proposed, in which a charge-to-voltage conversion efficiency can be changed so that the best S/N ratio can be obtained in accordance with the amount of incident light. As a distinctive configuration, a configuration is used, in which a plurality of horizontal transfer registers are provided, in which each of charge detection circuits having charge-to-voltage conversion efficiencies that are different from each other is connected to a corresponding one of the horizontal transfer registers, and in which the horizontal transfer registers are selectively used in accordance with the amount of received light.
Furthermore, in Japanese Unexamined Patent Application Publication No. 8-298626, a CCD solid-state image pickup device is proposed, which has both a typical-sensitivity mode and a high-sensitivity mode so that the CCD solid-state image pickup device can be used both during the daytime and during the nighttime. As a distinctive configuration, a configuration is used, in which a horizontal transfer register has divided output terminals of two transfer channels, in which each of the output terminals is connected to a corresponding one of a charge detector having a typical sensitivity and a charge detector having a high sensitivity (FD amplifiers), and in which a signal in the typical-sensitivity mode and a signal in the high-sensitivity mode that are output from the respective charge detectors are selectively output via a sensitivity switching circuit.
In Japanese Unexamined Patent Application Publication No. 2003-158679, a CCD solid-state image pickup device that is utilized in an endoscope in which either a typical image pickup mode or a fluorescence image pickup mode can be set is proposed. As a distinctive configuration, the following configuration is used. A charge multiplication stage having a plurality of stages is connected to an output side of a horizontal transfer register. A floating gate electrode is formed in the middle of the charge multiplication stage. Signal charge that has been output from the charge multiplication stage is supplied to a first floating diffusion amplifier (FDA) having a low conversion coefficient. An intermediate output signal of the charge multiplication stage is supplied from the floating gate electrode to a second FDA having a high conversion coefficient. Then, an output signal of the first or second FDA is selected via a selector, thereby selecting an image signal in the fluorescence image pickup mode (a first FDA output) or an image signal in the typical image pickup mode (a second FDA output).